The present invention relates generally to seismic data acquisition and analysis through conducting a seismic survey, and more particularly to methodologies for improving the accuracy of the results based upon the seismic data analysis.
The generation and recording of seismic data involves many different receiver configurations, including laying geophones or seismometers on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel, suspending hydrophones vertically in the sea or placing geophones in a wellbore (as in a vertical seismic profile) to record the seismic signal. A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data. Optimal acquisition varies according to local conditions and involves employing the appropriate source (both type and intensity), optimal configuration of receivers, and orientation of receiver lines with respect to geological features. This ensures that the highest signal-to-noise ratio can be recorded, resolution is appropriate and extraneous effects such as air waves, ground roll, multiples and diffractions can be minimized or distinguished, and removed through processing.
Underwater seismic exploration is widely used to locate and/or analyze subterranean geological formations for the presence of hydrocarbon reservoirs. One type of survey uses a boat towing a plurality of air guns and an array of ocean bottom nodes (OBN) placed on the ocean floor. The nodes are placed on the ocean floor by means of a remote operated vehicle (ROV) and subsea loader; typically the ROV and subsea loader are deployed from a deployment/retrieval boat separate from the tow or gun boat.
To acquire the data, compressed air shots are released from the air guns at known periodic intervals and the location and timing of each shot is recorded. Likewise, the timing and intensity of the nodes sensing of each compressed air shot is recorded. The data is typically collected for at least 30 days to cover one full tidal cycle, but may be collected over a longer period depending on, for example, the size of the area being surveyed.
Four-dimensional seismic data collection generally comprises three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4D seismic data is one of several forms of time-lapse seismic data. Such data can be acquired on the surface or in a borehole. Time lapse seismic data involves seismic data collection from the surface or a borehole acquired at different times over the same area to assess changes in the subsurface with time, such as fluid movement or effects of secondary recovery. The data are examined for changes in attributes related to expressions of fluid content. Time-lapse seismic data can repeat 2D, 3D (which is known as 4D seismic data), crosswell and VSP (vertical seismic profile) data.
Increased use of OBN acquisition for deep water time lapse monitoring has shown a need to have very accurate shot and receiver positions, as well as a good understanding of the high frequency water column velocity and height variations during acquisition that affect seismic event timing. As mentioned, the nodes are typically on the seafloor in excess of thirty days and thus observe at least one full cycle of tides and possibly large changes in water column velocity. The tide cycles are problematic because they affect the vertical distance between the nodes and shots during acquisition. High frequency velocity variations must be understood in order for water column statics corrections to be computed to correct the data to a single water depth dependent velocity for downstream processing and imaging of the OBN data.
Ocean bottom nodes all have internal clocks that are independent of the master clock (typically GPS time), which serves as the reference for all shots in a given survey. Each internal clock experiences some amount of clock drift, due to factors such as crystal oscillator aging, human error in the calibration, and simply delayed activation as compared to the master GPS clock, during the deployment of the node. For each node, the clock drift value is measured when the node is synced with the master clock upon retrieval of the node from the seafloor. However, there are instances where this measurement cannot be made or is made unreliably, which leads to uncertainty in the clock drift measurement. To determine clock drift in nodes with unreliable field measurements, spatially neighboring shot traces with significantly different time of acquisition are compared and a clock drift is chosen that removes discontinuities in the direct arrival. However, if there is a corresponding change in water velocity, due to changes in acquisition time, then a discontinuity analysis can be deceiving or lead to incorrect drift adjustments.
A process that corrects for only one of shot positioning, receiver positioning, receiver timing, or water column velocity will be subject to leakage of errors in the other dimensions. A technique must be used that simultaneously solves for source position, receiver position, receiver clock drift, and water velocity as a function of acquisition time.
It is a primary object and advantage of the present invention to provide a method, system, and product that will solve shot and receiver positions, receiver timing, and the water column velocity model simultaneously.
Other objects and advantages of the present invention will in part be apparent to those of skill in the art, and in part appear hereinafter.